Due to excellent mechanical properties, thermal properties and electric properties, polycarbonate resins are used as a raw material of a molded product in various industrial fields. In recent years, polycarbonate resins are widely used in functional products that utilize in combination optical properties of polycarbonate resins.
With such an expansion of application fields, properties required of polycarbonate resins have come to be diversified. Polycarbonate resins prepared by using as raw materials 2,2-bis(4-hydroxyphenyl)propane or 1,1-bis(4-hydroxyphenyl)cyclohexane, that have conventionally been used, often cannot meet such request sufficiently. Therefore, polycarbonate resins having various chemical structures have been proposed in accordance with the application and required properties.
As products utilizing electric and optical properties of polycarbonate resins, an organic electrophotographic photoreceptor utilizing a polycarbonate resin as a binder resin for a functional material such as a charge-generating material and a charge-transporting material can be given.
An organic electrophotographic photoreceptor is required to have prescribed sensitivity, electric properties and optical properties according to electrophotographic processes.
In the case of an organic electrophotographic photoreceptor, operations such as corona discharge, toner development, transfer to paper, cleaning or the like are conducted repeatedly on the surface of the photosensitive layer thereof. Therefore, whenever these operations are conducted, an electric or mechanical external force is applied to the surface of the photosensitive layer. Therefore, in order to maintain the image quality of an electrophotography over long periods, the photosensitive layer provided on the surface of an electrophotographic photoreceptor is required to have resistance to these external forces.
Further, an organic electrophotographic photoreceptor is normally produced by a method in which a binder resin is dissolved in an organic solvent together with a functional material, and the resulting mixture is formed into a film on a conductive base by casting, and hence, it is required to have solubility and stability for an organic solvent.
Conventionally, as a binder resin for a photoreceptor, a polycarbonate resin prepared by using, as a raw material, 2,2-bis(4-hydroxyphenyl)propane or 2,2-bis(4-hydroxyphenyl)cyclohexane has been used. These resins do not have sufficient durability. Therefore, in order to meet these requirements, various techniques have been developed.
As an effective technology for improving wear resistance of the photosensitive layer, use of a polycarbonate copolymer is known.
For example, a resin disclosed in Patent Document 1 is produced by copolymerizing a bisphenol Z skeleton having good solubility with biphenol having wear resistance. However, the amount of the biphenol component is at most about 23 mol % in terms of copolymerization ratio since the polycarbonate oligomer as the raw material is a di- to tetramer. The resin disclosed in Patent Document 1 does not have sufficient wear resistance, and hence, further improvement in wear resistance has been required.
Patent Documents 2 and 3 each disclose a polymer in which the copolymerization ratio of the skeleton having excellent wear resistance such as biphenol (that is produced from a raw material of which the number of monomers in an oligomer is reduced) has been increased to 25 to 47 mol %, and report that this polymer has significantly excellent wear resistance.
However, as for mechanical deterioration that occurs at the time of using toner containing a large amount of an external additive (filming, for example), the resins disclosed in Patent Documents 2 and 3 cannot sufficiently prevent. On the other hand, in these resin, the surface of a photoreceptor cannot be worn easily, there is a concern that whitening, black spots, streaks, insufficient cleaning or other phenomenon may occur.
Toner filming is a phenomenon in which an external additive in a toner is adhered to the outermost surface of the photosensitive layer, and the resin components in the toner are firmly adhered thereto with the portion where the additive is adhering being as the center. As a conventional means for solving this problem, a method in which wear resistance of a photoreceptor is lowered, whereby the surface on which filming occurs can be peeled off easily, or a method in which addition of a toner is suppressed by lowering the surface free energy of the drum surface, or other methods were conceived.
However, as for the former method, there is a problem that the drum durability is deteriorated and, as for the latter method, there is a problem that the surface properties cannot be maintained. Accordingly, it was difficult to produce a photoreceptor having excellent wear resistance and suffering no toner filming (mechanical deterioration).